Arc discharge lamps such as fluorescent lamps which employ a phosphor within a discharge chamber, which chamber also contains an ionizable medium together with mercury vapor, suffer from a gradually decreasing light output as they age. Various factors contribute to the drop-off in light output during operation, and some of these may be caused by deposits of impurities from the cathode; the formation of various compounds of mercury; changes in the phosphor itself; and changes in the glass envelope, particularly where it may be subject to ultraviolet radiation. The ability of such lamps to resist drop-off in light output is generally termed lumen maintenance, and it is measured as the ratio of light output at a given life span compared to an initial light output and expressed as a percentage. Since the light output of a new lamp is apt to vary considerably until it has been in operation for some time, it is usual to start lumen maintenance measurements from some time other than time zero.
While decreasing light output with time is an occurrence for all fluorescent lamps, it is much more of a problem for high output and very high output lamps than it is for normally loaded lamps.
The art and artisians of lamp design have expended much time and money in an effort to solve these problems. Although the problem of lumen maintenance still exists, it has been alleviated to some extent. Many of the solutions proposed involve the employment of refractory metal oxides as protective agents against the hostile environment which exists in arc discharge devices.
For example, U.S. Pat. No. 3,067,356 teaches the use of refractory oxides such as Al.sub.2 O.sub.3, SiO.sub.2 and TiO.sub.2 as coatings on the interior glass surface of fluorescent tubes. U.S. Pat. No. 3,514,276 teaches the use of alumina (Al.sub.2 O.sub.3) as a thermal ray reflecting film. U.S. Pat. No. 3,541,377 teaches the application of an alumina coating to the interior surface of a fluorescent tube through the application of boehmite (AlO(OH)) and subsequent processing. U.S. Pat. No. 3,599,029 teaches the application of a titanium dioxide layer having a layer of aluminum oxide thereover. U.S. Pat. No. 3,748,518 lumen maintenance improvement by doping fluorescent lamp glass with titania and applying a layer of TiO.sub.2 thereover by vapor deposition. U.S. Pat. No. 3,847,643 relates to the treatment of fluorescent lamp tubing with aluminum and titanium containing compounds for improving phosphor maintenance. U.S. Pat. No. 3,890,530 relates to two-layer precoats on fluorescent lamp tubing, i.e., a layer of aluminum oxide over a layer of titanium dioxide. U.S. Pat. No. 3,967,153 teaches a fluorescent lamp having an inner, transparent electrically conductive coating thereon having a protective coating of finely powdered aluminum oxide thereover. U.S. Pat. No. 4,058,639 relates to a process of manufacturing fluorescent lamps having alumina coatings on the interior of the glass envelope. U.S. Pat. No. 4,079,288 teaches an ultra-violet reflecting underlayer of alumina particles in fluorescent lamps.
U.S. Pat. No. 2,386,277 teaches a sputtered coating of alumina over a phosphor layer in fluorescent lamps. U.S. Pat. No. 3,886,396 teaches a porous, discontinuous layer of alumina of relatively light weight over a phosphor layer and U.S. Ser. No. 228,865, filed Jan. 27, 1981, and assigned to the assignee of the instant application teaches the application of a relatively heavy layer of alumina over a phosphor layer. U.S. Pat. Nos. 3,995,191 and 3,995,192 also disclose aluminum oxide layers over phosphor layers to improve maintenance.
U.S. Pat. No. 2,331,306 discloses a fluorescent lamp wherein aluminum oxide is mixed with a phosphor to promote adherence to the glass. U.S. Pat. No. 3,887,725 discloses the addition of zinc orthophosphate to a calcium halophosphate phosphor slurry as a means of increasing lumen maintenance.
Recent improvements in fluorescent lighting have included lamps containing multiple layers of different phosphors. Where one of the phosphors has a known, better maintenance than the other, it has been suggested that the former be applied over the latter to at least partially shield the latter from the hostile environment within the lamp.
It would be an advance in the art if a more favorable means could be found for increasing the lumen maintenance of multiple phosphor layer fluorescent lamps.